The present invention relates to an image forming apparatus which develops a static charge image and a magnetic latent image in electrophotography, electrostatic printing, magnetic recording and others, and more particularly to an image forming apparatus using a heat fusing scheme such as heat roller fusing.
A cleanerless process which does not have a cleaner such as a blade on a photoreceptor surface is a technique which is advantageous to a reduction in size of an apparatus or conservation of a toner, and various inventions of the cleanerless process have been disclosed. For example, U.S. Pat. No. 4,727,395 discloses a development/simultaneous cleaning technique in an reversing phenomenon. In particular, this technique is also effective in realization of full color in recent image forming apparatuses and has begun to be adopted in a four-drum tandem mode, and, e.g., Japanese Patent No. 3342217, Jpn. Pat. Appln. KOKAI Publication No. 7-64366 and others disclose examples concerning the cleanerless process having this tandem configuration.
As merits of the cleanerless process, there are simplification of a configuration since a photoreceptor cleaner is not necessary, realization of long duration of life of a photoreceptor because the photoreceptor is not scraped by a cleaner, an improvement in toner consumption efficiency since a post-transfer residual toner is recovered by a developing device for recycling, and others. However, considering a color configuration of the tandem mode, there is a problem that reverse transfer from a color station on a preceding stage to a color station on a subsequent stage occurs and a mixed color is generated depending on an image to be printed, thereby resulting in a change in hue.
This reverse transfer is generated when a yellow toner is transferred onto a transfer medium such as a paper sheet or an intermediate transfer body in, e.g., a first transfer station and then the transfer medium passes through a transfer station for, e.g., a cyan color on a subsequent stage. At this time, if a condition that a cyan toner is also transferred is provided, the cyan toner is transferred onto the yellow toner on the transfer medium. However, if a condition that the cyan toner is not transferred onto a part where the yellow toner has been already transferred on the transfer medium is combined, a part of the yellow toner on the transfer medium disadvantageously adheres to a photoreceptor of a cyan station. Further, the reverse-transferred yellow toner is collected in a developing device of the cyan station since the photoreceptor of the cyan station is not provided with a cleaner. When this process is repeated, a hue of a developing agent in the cyan developing device gradually changes, and a so-called mixed color is generated.
That is, in order to avoid generation of a mixed color, it is important to prevent reverse transfer from occurring, and it is known that this reverse transfer can be improved by enhancing mold release properties of a photoreceptor or reducing the adherence between a photoreceptor and a toner by, e.g., adopting a spherical toner which can decrease a contact surface. Furthermore, reverse transfer is greatly affected by an electric discharge phenomenon, and a reverse transfer generation quantity is increased when the transfer bias is high whilst this generation quantity is decreased when the transfer bias is lowered. In a regular process, when the transfer bias is lowered to a level at which reverse transfer hardly occurs, a transfer electric field is insufficient and the post-transfer residual toner is increased. However, by combining the photoreceptor with the excellent mold release properties, the spherical toner and others, it is possible to obtain transfer efficiency which has no problem from a practical standpoint even if the bias is lowered to a level at which reverse transfer hardly occurs.
Moreover, since the reverse transfer phenomenon is greatly affected by electric discharge, there has been proposed a method which reduces the reverse transfer phenomenon by contriving a configuration of a transfer section. Since the electric discharge which generates reverse transfer is produced in a small air gap between a part where a non-image section of a photoreceptor moves close to a transfer medium which supports a toner of a station on a preceding stage and a part where the non-image section moves apart from the toner, the above-described method lowers an electric field in a state where the both members come close to each other or move away from each other to a level at which no electric discharge is generated on both the inlet side and the outlet side of a contact nip of the both members. By doing so, the reverse transfer can be considerably reduced. However, even in this state, if the both members are in contact with each other in the transfer nip, the electric discharge is generated and the reverse transfer occurs when the adhesion of the both members is not stabilized. Additionally, when the transfer nip is broadened in such a configuration, an advantage can be obtained if the adhesion is stably achieved, but the reverse transfer or an image disturbance is apt to be excessively produced if the adhesion becomes unstable due to a small blur or foreign particles.
Here, as factors which make the adhesion unstable, the following can be considered. For example, when two-component development is used, a developing agent comprises a toner and a carrier. A particle diameter of the toner is approximately 3 to 12 μm and, on the other hand, a particle diameter of the carrier is approximately 20 to 80 μm.
In a usual state, the carrier electrizes and carries the toner in the developing device, forms an ear by a magnetic force in a state where the toner is supported on a developing roller, and then returns to the inside of the developing device for circulation. The carrier stays in the developing device and does not adhere to the photoreceptor in the normal state. However, the carrier has an operating life, and its coercive force may be weakened and the carrier may partially adhere to the photoreceptor when the carrier is deteriorated in some cases, for example. In this example, since the particle diameter of the carrier is larger than that of the toner, the carrier is intervened between the transfer medium and the photoreceptor in the transfer nip, and the adhesion between the photoreceptor and the transfer medium thereby becomes unstable so that an air gap is produced, thereby generating abnormal electric discharge. That is, with generation of the abnormal electric discharge, a reverse transfer quantity is increased. Additionally, in case of the cleanerless process, since the carrier cannot be readily collected by a regular cleaner or the like when the carrier adheres to the photoreceptor, an influence of the carrier is serious as compared with a process using a cleaner. That is, the carrier may repeatedly stay on the photoreceptor and keep adversely affecting in the transfer section in some cases, and a mixed color due to the reverse transfer as well as an image defect such as transfer irregularities can be saliently generated.
Further, the adhesion is also affected by a step between a part where the toner is developed and a part where the toner is not developed.
For example, in cases where printing is performed in each solid color and a thickness is set to 15 μm, an air gap of 15 μm is generated between an image section and a non-image section of a magenta image in a magenta station at a subsequent stage with a yellow toner being developed on a transfer medium. Furthermore, if the same pattern as that in the magenta station is obtained in a cyan station on a subsequent stage, a step between the transfer medium which supports an yellow image in the cyan station and the non-image section in the cyan station is 30 μm at maximum. As described above, a step generated due to presence/absence of the toner leads to the unstableness of the adhesion between a belt and the photoreceptor and generation of an air gap, and abnormal electric discharge is thereby produced, resulting in an increase in reverse transfer quantity.